[OFDM]
In data transmission by OFDM (Orthogonal Frequency Division Multiplexing), a large number of orthogonal subcarriers are used in the transmission band, and data is allocated to the amplitude and the phase of each of the subcarriers. Data of each symbol called an OFDM symbol is transmitted. At the time of transmission, an IFFT (Inverse Fast Fourier Transform) is performed on each OFDM symbol.
FIG. 1 is a diagram showing OFDM symbols. An OFDM symbol is normally formed with an effective symbol that is a signal interval in which an IFFT is performed at the time of transmission, and a guard interval (GI) formed by copying the waveform of part of the latter half of the effective symbol and placing the copy at the top of the effective symbol.
As the guard interval is formed at the top of the OFDM symbol, resistance to multipathing can be increased. Such OFDM symbols constitute one OFDM transmission frame.
[Signaling of DVB-T2]
DVB-T2 (the second-generation digital terrestrial broadcasting standards in Europe) are digital terrestrial broadcasting standards using OFDM. According to DVB-T2, frames called T2 frames are defined, and data is transmitted by the T2 frame.
Each T2 frame contains two kinds of preamble signals called P1 and P2, and those preamble signals contain information necessary for operations such as OFDM signal demodulation.
FIG. 2 is a diagram showing the frame structure of a T2 frame. As shown in FIG. 2, one T2 frame includes a P1 symbol, P2 symbols, and data symbols (Normal or FC) in this order.
The P1 symbol is the symbol for transmitting P1 Signaling, and contains the following information a through d.
a. Frame identification
b. Transmission method
c. FFT size
d. Partial GI length
The frame identification indicates whether the transmission frame is a T2 frame or a FEF (Future Extension Frame). The transmission method indicates whether the transmission method is SISO (Single Input, Single Output) or MISO (Multiple Input, Single Output). The FFT size indicates the number of points in one IFFT operation on the transmission side. The partial GI length indicates to which group the GI length being used in the symbol transmission belongs, with seven types of GI lengths being divided into two groups. That is, only the GI length group can be recognized from P1 Signaling, and, to recognize the GI length, L1PRE Signaling contained in a P2 symbol needs to be decoded.
The P2 symbols are the symbols for transmitting L1PRE Signaling and L1POST Signaling. Hereinafter, L1PRE Signaling will be referred to as L1PRE, and L1POST Signaling will be referred to as L1POST, where appropriate. L1PRE contains the information necessary for decoding L1POST, and the following control information e through g, which are necessary for demodulating a data symbol. L1POST contains parameters necessary for a receiver to access a physical layer.
e. GI Length
f. Time for known signal called Scattered Pilot (SP)/the interval pattern in the frequency direction
g. The number of symbols in one frame
If the information e through g of L1PRE can be decoded, the receiving device can estimate the channel characteristics by using the SP, and can perform data symbol equalization by performing interpolation in the temporal direction/the frequency direction.
As FFT sizes of the P2 symbols, the six types of sizes, 1 K, 2 K, 4 K, 8 K, 16 K, and 32 K, are defined. As GI lengths of the P2 symbols, the seven types of sizes, 1/128, 1/32, 1/16, 19/256, ⅛, 19/128, and ¼, are defined. The same FFT size and the same GI length as the FFT size and the GI length of the symbols (Normal, FCS) other than P1 and P2 are selected for the P2 symbols.